1. Field of the Invention
The present invention relates to a manufacturing method of a laser diode ("LD") and a laser diode array ("LD array"), and more particularly, to a manufacturing method of a LD and a LD array in which production yield is greatly improved by an epitaxy step.
2. Description of the Related Art
Generally, it is necessary to stabilize a lateral mode and lower the operating current in a LD. The current is usually concentrated in a narrow region. For example, a current blocking layer is formed on the left and right of a V-channel. This LD structure is called an inner stripe-shaped configuration. Two epitaxy steps are required to grow the LD structure. The current blocking layer is grown on a substrate, and a channel is formed by etching as a first epitaxy step. Next, a double heterojunction structure is grown as a second epitaxy step.
A LD array is utilized to increase the intensity of an output base. The LD array outputs a stable beam by operating in a phase-locked state in which respective LDs are strongly coupled. There is an evanescent mode coupling by an index guide, a leakage mode coupling by an anti-guide, and a gain guide coupling (and the like) as coupling modes between respective LDs.
In the evanescent mode coupling, the effective index of a lasing region of a unit LD for generating a laser beam is bigger than that of a coupling region between neighboring LDs, so the coupling between unit LDs is not strong. On the other hand, in the leakage mode, the effective index of the coupling region is bigger than that of the lasing region, so the coupling between unit LDs is strong.
Two-step epitaxy layers have more crystalline defects because the layers are fabricated after exposure to air and etching. To solve this problem, a method for growing the LD by one-step liquid phase epitaxy (hereinafter termed "LPE") was suggested by K. Kishino, et al., Japanese Journal of Applied Physics, pp. L 473-L 475, July, 1983.
FIGS. 1A to 1D are sectional views showing a conventional manufacturing process of a LD by one-step LPE.
As seen from FIG. 1A, a predetermined part of N-type GaAs semiconductor substrate 11 having a (100) crystal plane is forwardly mesa-etched in a &lt;011&gt; direction.
As seen from FIG. 1B, first and second current blocking layers 12 and 13 of N- and P-type Al.sub.x Ga.sub.1-x As, respectively, are fabricated. The first current blocking layer 12 is not formed on top of the mesa-like structure 11a on the semiconductor substrate 11, but formed only to the left and right of the mesa-like structure 11a.
As seen from FIG. 1C, the second blocking layer 13 is contacted with unsaturated melted material so as to form a V-channel 14 having a &lt;01l&gt; direction by melt etching in situ. If the second current blocking layer 13 on the mesa-like structure surface is removed and the mesa-like structure is exposed, the semiconductor substrate 11 is etched faster than the second current blocking layer 13 to form the V-channel 14, because the melting rate of GaAs is generally 10-15 times faster than that of AlGaAs.
As seen from FIG. 1D, a first cladding layer 15 of N-type Al.sub.y Ga.sub.1-y As, an active layer 16 of N- or P-type Al.sub.z Ga.sub.1-z As, a second cladding layer 17 of P-type Al.sub.y Ga.sub.1-y As, and a cap layer 18 of P.sup.+ -type GaAs are sequentially deposited on the surface of the second current blocking layer 13. At this time, the first cladding layer 15 is grown faster at the V-channel 14 and finally forms a flat upper surface.
The refractive index of the active layer 16 is chosen to be greater than that of the first and second cladding layers 15 and 17 so that generated light is limited to the active layer 16. Moreover, the refractive index of the first cladding layer 15 is chosen to be greater than that of the first and second current blocking layers 12 and 13 so that the light generated from the active layer 16 is concentrated in the V-channel 14. Accordingly, x, y and z, representing the content of aluminum, should satisfy the relationship 1.gtoreq.x&gt;y&gt;z.gtoreq.0. N-type and P-type electrodes 19 and 20 are formed on the exterior surfaces of the cap layer 18 and the semiconductor substrate 11, respectively.
In the above described method, the amount of material used in manufacturing the LD is controlled. A V-channel is formed by melt-etching in situ by virtue of the unsaturated melted material, and then the epitaxy step is performed without exposure to air, which prevents generation of crystalline defects.
A LD array having a leakage mode coupling structure is disclosed in Appl. Phys. Lett., 53(6), pp. 464-466, Aug. 1988.
FIGS. 2A to 2C are sectional views showing a manufacturing method of a LD array of a conventional leakage mode coupling structure.
As seen from FIG. 2A, a first current blocking layer 22 of N-type Al.sub.y Ga.sub.1-y As and a second current blocking layer 23 of N-type Al.sub.x Ga.sub.1-x As are sequentially formed on the surface of a P.sup.+ -type GaAs substrate 21 by LPE or metal organic chemical vapor deposition (hereinafter termed "MOCVD"). The first and second current blocking layers 22 and 23 are then etched to a predetermined depth to form a plurality of channels 24.
As seen from FIG. 2B, a first cladding layer 25 of P-type Al.sub.y Ga.sub.1-y As is formed above the second current blocking layer 23 to entirely cover the channels 24. An undoped active layer 26 of I-type Al.sub.z Ga.sub.1-z As, a second cladding layer 27 of N-type Al.sub.y Ga.sub.1-y As, and a cap layer 28 of N.sup.+ -type GaAs are then sequentially fabricated on the surface of the first cladding layer 25. The first cladding layer 25 is grown faster in the channels and forms a flat upper surface.
The aluminum mole concentration of the first cladding layer 25 should be greater than that of the second current blocking layer 23 so that the effective index of the coupling region is greater than that of the lasing region. That is, the constant y should be greater than the constant x. The effective index of the active layer 26 is chosen to be greater than that of the first and second cladding layers 25 and 27 so that the generated light is limited to the active layer 26. Accordingly, the constants y and z should satisfy 1.gtoreq.y&gt;z.gtoreq.0.
Referring to FIG. 2C, after SiO.sub.2, Si.sub.3 N.sub.4, or the like is deposited on the surface of the cap layer 28, insulating layers 29 are formed at end portions of the cap layer 28 by conventional photolithography. Subsequently, N-type electrode 30 is formed on the upper exterior surface of the cap layer 28, and P-type electrode 31 is formed on the lower exterior surface of the semiconductor substrate 21.
The LD array described above is operated in a leakage mode in a state where the aluminum mole concentration of the second current blocking layer 23 is greater than that of the first cladding layer 25, and the effective index of the coupling region is greater than the lasing region.
In the conventional LD described above, the convexity of a given mesa-like structure is not high in topographical aspect because the mesa-like structure is forwardly formed, and therefore the second current blocking layer is thickly formed on the surface of the mesa-like structure due to a property of LPE growth. Further, the unsaturated level of the melting material is controlled by an amount of the material, and therefore the unsaturated level of the material is not uniform due to mass variation, etc. at the time of purification and etching of the material. The unsaturated level becomes higher to remove the second current blocking layer formed on the mesa-like structure, but the unsaturated level is not uniform, thereby diminishing the reliability of melt-etching.
Further, in the conventional LD array described above, the channel is formed by the etching between the first epitaxy step and the second epitaxy step, thereby oxidizing the etched surface or causing defects at the surface. Accordingly, the quality of layers grown by the second epitaxy step are less than optimal, diminishing the reliability of the device.